1. Field of Application
The present invention relates to a method for the controlled break-up of liquid jets for the production of substantially monodispersed drops.
In the following description and the subsequent claims, with the term: xe2x80x98substantially monodispersed dropsxe2x80x99, it is intended to mean essentially spherical drops of liquid of substantially identical diameter.
The present invention also relates to a device for implementation of the above mentioned method.
As known, in the field of prilling of melted materials in more or less viscous liquid form, e.g. for the production of fertilizers, the requirement to provide a method of controlled break-up of liquid jets which on the one hand would allow obtaining substantially monodispersed drops and on the other hand would be reliable, easy to implement and not requiring high energy consumption and operating costs is becoming more urgent.
In the following description, with the term: xe2x80x98prillingxe2x80x99, it is intended to mean a process by which a melted material in liquid state is made to pass through a plurality of holes to form corresponding liquid jets which break up into a plurality of drops which solidify into solid granules upon cooling. Generally, the solidification takes place by free fall of the drops in counter-current against an air flow in a suitable device called prilling tower.
This process is based on the phenomenon according to which a liquid jet breaks up into a plurality of drops due to dynamic instability resulting from its surface tension and specifically whenever the amplitude of oscillation of the liquid jet increases until it becomes equal to the radius of the jet.
The factors at the origin of this dynamic instability of the jet can be external, e.g. disturbances caused by friction with air, or internal such as disturbances due to turbulence of the liquid.
2. Prior Art
In order to meet the above mentioned requirement, there have been proposed methods which make it possible to obtain substantially monodispersed drops through the controlled break-up of a liquid jet to which is transmitted a disturbance of predetermined magnitude in such a manner that the wavelength of oscillation of the jet is greater than its circumference.
Indeed, it has been found possible to obtain homogeneous and regular break-up of a liquid jet by transmitting thereto an oscillation of a wavelength substantially between the following values:
7*r less than lambda less than 14*rxe2x80x83xe2x80x83(1)
where r is the radius of the liquid jet and lambda is its wavelength (see: J. M. Schneider and C. D. Hendricks, xe2x80x9cSource of Uniform Liquid Dropletsxe2x80x9d, Review of Scientific Instruments, Vol. 35, No. 10, 10/1964).
In general, according to the theory of C. Weber (see for example, xe2x80x9cAtomization and Spray Dryingxe2x80x9d, chapter 1, W. R. Marshall Jr., Chem. Eng. Progr. Monogr. Series, no. 2, vol. 50, 1954), the wavelength of oscillation most effective for obtaining controlled break-up of a liquid jet and hence substantially monodispersed drops is determined by the following formula.
lambda/r=8.886*(1+3Z)xc2xdxe2x80x83xe2x80x83(2)
where r is the radius of the liquid jet, lambda is its wavelength and Z is the ratio of the square root of Weber""s number to Reynolds"" number (Z=Wexc2xd/Re).
The methods according to the prior art provide this controlled break-up by changing with a predetermined frequency the flowrate of the jets forming liquid in such a manner as to transmit to the jets an oscillation of desired wavelength.
As an alternative, the disturbance responsible for a controlled break-up of the liquid jets is imparted according to the above mentioned methods by means of:
acoustic vibrations of predetermined frequency produced by a sound source and transmitted through the air to the liquid jets leaving a perforated surface;
acoustic vibrations of predetermined frequency produced by a sound source and transmitted in the air or in the gas standing over the head of the jets forming liquid contained in a suitable perforated basket;
vibrations transmitted directly to the jets forming liquid by means of vibrators, acoustic sources or rotating valves;
mechanical vibrations appropriately transmitted to the perforated basket for jets formation or parts thereof.
Methods of this type are described e.g. in EP-A-0 233 384, EP-A-0 320 153 and U.S. Pat. No. 4,585,167.
However, the above mentioned methods display a series of drawbacks, the first of which is that they are difficult to implement and unreliable.
Indeed, to be effectively implemented, these methods necessitate sophisticated equipment complicated to manufacture and requiring constant maintenance.
Consequently, to appropriately control the break-up of the liquid jets, high construction, operating and maintenance costs are necessary.
In addition, because of their complexity, mainly due to the need to provide means of transmission of acoustical or mechanical vibrations, the equipment for implementation of the methods according to the prior art cannot ensure obtaining substantially monodispersed drops in a constant manner over time.
The technical problem underlying the present invention is to make available a method for the controlled break-up of liquid jets allowing obtaining substantially monodispersed drops and simultaneously which would be reliable, easy to implement and not requiring high energy consumption and operating costs.
According to the present invention the above mentioned problem is solved by a method of the type indicated above and comprising the following steps:
feeding a plurality of first liquid jets having stationary motion to a plurality of mutually independent sectors defined in a perforated surface;
forming of a head of liquid in these sectors near the perforated surface;
causing the liquid to flow across the perforated surface to form a plurality of second liquid jets;
periodically changing with a predetermined frequency the momentum of the liquid fed to a predetermined sector so as to impart to the liquid present in that sector a disturbance of predetermined magnitude consisting of a periodic change of the pressure near the perforated surface which is transmitted to the second liquid jets causing their controlled break-up in a plurality of substantially monodispersed drops.
In the following description and subsequent claims, with the term: xe2x80x98liquid jet having stationary motionxe2x80x99, it is intended to mean liquid jets having flowrate and velocity constant in time.
In the following description and subsequent claims, with the term: xe2x80x98mutually independent sectorsxe2x80x99, it is intended to mean sectors hydraulically separate from each other.
Advantageously, by means of the present invention it is possible to obtain substantially monodispersed drops by feeding the liquid for formation of the second jets under stationary conditions and at the same time causing variation in an appropriate manner of the pressure of this liquid.
This is made possible in particular by division of the perforated surface in a plurality of sectors and by feeding to the sectors a plurality of first jets.
Indeed, by proceeding in this manner it was surprisingly found that it is sufficient to appropriately change the momentum of the liquid fed to a predetermined sector, while holding constant the momentum of the first jets fed to the perforated surface and thus operating under stationary conditions, to obtain in a simple but at the same time accurate and reliable manner a controlled break-up of the second liquid jets leaving the perforated surface.
Advantageously, the change in the momentum of the liquid fed to a predetermined sector takes place periodically and with time intervals such as to transmit to the second liquid jets an effective disturbance of a wavelength greater than their circumference and preferably between the values indicated in the above formula (1).
Thanks to the present invention, it is possible to utilize in a simple and effective manner the flowratexe2x80x94under stationary conditionsxe2x80x94of the liquid fed to the perforated surface for the formation of the second jets in order to obtain a controlled break-up of the latter without the drawbacks deriving from the methods according to the prior art.
According to a preferred embodiment of the present method, the sectors are side-by-side and the momentum of the liquid fed to a predetermined sector is different from the momentum of the liquid fed to a sector adjacent thereto.
According to this embodiment, the momentum of the liquid fed to each sector is advantageously changed by imparting to the perforated surface a relative motion with respect to a liquid distributor provided upstream of said surface for the delivery of the first jets.
The relative motion is preferably carried out by moving the perforated surface with respect to the liquid distributor while holding constant the distance between them.
Preferably, the effective disturbance transmitted to the second jets is generated by changing periodically with a predetermined frequency the number of first jets fed to a predetermined sector.
Advantageously, the first jets of liquid fed to the sectors are delivered with a substantially centrifugal flow by the distributor to the perforated surface which is substantially of tubular shape and is arranged externally and coaxially with respect to the distributor.
Preferably, the perforated surface of substantially tubular shape; is rotated around its axis and the sectors extend longitudinally in this surface.
In this manner it is possible to distribute radially the second jets to be broken up to obtain a xe2x80x98rainxe2x80x99 at 360xc2x0 of substantially monodispersed drops which depart radially from each other.
Preferably, the first liquid jets fed to the sectors intersect orthogonally the related liquid head in such a manner as to avoid formation of secondary turbulence near the perforated surface which would negatively influence the controlled break-up of the second jets.
In accordance with the method according to the present invention, the wave function obtained from periodic change in the momentum of the liquid fed to each sector is preferably of the sinusoidal type so as to avoid the presence of second oscillations within the liquid.
According to another aspect of the present invention, there is also made available a device for the break-up of liquid jets for the production of substantially monodispersed drops comprising:
a first liquid distributor comprising a perforated wall for delivering a plurality of first liquid jets having stationary motion;
a second liquid distributor supported at a predetermined distance from the first distributor and divided in a plurality of chambers in fluid communication with the first jets and of equal volume and mutually independent and provided with a perforated wall substantially parallel with the perforated wall of the first distributor for formation of a plurality of second liquid jets;
with the first and second distributors having relative motion with respect to each other so as to vary periodically with predetermined frequency the momentum of the liquid fed to a predetermined chamber while imparting to the liquid present in this chamber a disturbance of predetermined magnitude consisting of a periodic change in the pressure transmitted to the second liquid jets to cause controlled break-up thereof into a plurality of substantially monodispersed drops.
The characteristics and advantages of the invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.